The invention concerns a twin-wire web former in a paper machine comprising a carrying wire and a covering wire, which together form a twin-wire forming zone, in which a forming unit is fitted. The forming unit comprises a forming board and a drainage box placed one opposite the other. The drainage box comprises a number of ribs and spaces between these ribs through which water is drained out of a web running between the carrying and covering wires. The water drains out through the spaces between these ribs to a significant extent by the effect of negative pressure into the drainage box.
With increasing running speeds of paper machines, several problems in the web formation have been manifested with more emphasis. The phenomena, such as centrifugal forces which affect both the fiber mesh in the former of the paper machine and the water, which is still relatively free in connection with the fiber mesh, are usually increased in proportion to the second power of the web speed. The highest web speeds of most modern paper machines are of an order of about 1200 m/min. However, machines are being planned in which the intended web speed is as high as about 1700 m/min.
With respect to prior art related to the present invention, reference is made to U.S. Pat. No. 4,769,111, granted to A. Ahlstrom Corp.; to the assignee's FI Pat. Appl. No. 885609, and to FI Pat. Appls. Nos. 885606 and 885607 of Valmet-Ahlstrom Inc., wherein web formers marketed by the assignee under the trade mark "MB-Former" are described.
Further, reference is made to the assignee's FI Pat. Appls. 904489 (filed Sep. 12, 1990) and 911281 (filed Mar. 15, 1991), wherein various combinations of the above MB-former unit or units and hybrid and gap formers are described.
In the prior art MB-formers, the lower unit consists of a support board, which comprises relatively wide transverse ribs. Each of these ribs are loaded separately by means of a loading hose. The top side of the ribs is placed against the inner face of the lower wire and is provided with grooves running across the face in the longitudinal direction of the ribs. The function of the water gathered in these grooves is mainly to lower the friction between the lower wire and the ribs. It is a drawback of this prior art support board that the downward draining from the web is almost completely prevented. This is because water can drain only to a limited extent into the narrow and shallow grooves on the ribs. From these grooves the water can flow to the sides only.
In particular when a MB-unit or units are fitted, e.g., in accordance with the assignee's FI Pat. Appl. 904489, in a high-speed and wide gap former (the assignee's "HHS-former"=trade mark), the narrow and shallow grooves on the ribs in the support board are not adequate to remove the relatively large quantities of water. In the prior art MB-units, the draining takes place practically exclusively upwards towards the upper drainage box, aided by suction. Thus, in the prior art constructions, the formation of the web cannot be affected sufficiently by means of a MB-unit. This is because the fibrous web must be relatively dry when it arrives in the twin-wire forming zone in the MB-unit. As a result, the fibers in the web can no longer be displaced to a sufficient extent in relation to one another.
In the prior art, in the manufacture of certain paper grades, a coloring agent is added to the paper pulp. However, when the prior art MB-units are employed, this coloring agent is washed along with the water that is drained upwards, to a greater extent to the top face of the web. On the other hand, when attempts have been made in prior art former solutions to drain a higher proportion of water downwards to provide a better balance of color, the consequence has been inferior formation of the paper.